Padlocks typically have a single locking mechanism, but some have dual locking systems to further protect the lock against forced entry. For example, U.S. Pat. No. 5,044,180 to Lebrecht shows a round padlock with a round shackle. A plug and cylinder lock operates the shackle via a primary drive prong. A secondary lock latch also locks the shackle, and is disengageable therefrom by a cam which is rotatable by the key. U.S. Pat. No. 4,998,423 to Hsu shows a similar structure.
These locks, however, are still susceptible to attack especially from a prying force applied to the shackle. When a foreign object is inserted into the shackle and the shackle is forced open, shear stress is placed upon the primary drive prong, the secondary lock, and the cylinder. If the cylinder fails first, then the cylinder can be rotated, moving the primary drive prong, disengaging the secondary lock, and opening the lock.
Several patents show locks with breakable elements disposed between a key-operable lock and a bolt, for example U.S. Pat. Nos. 5,611,225 to Resch and 5,548,981 to Kirk, but a need exists for a padlock where the shear stress created by a foreign object being inserted into either the shackle or the cylinder will be carried by the primary drive prong, causing it to shear first, protecting the cylinder and preserving the integrity of the padlock.
Attempts at providing additional protection from attack led to the use of protective shells placed over the padlock. For example, U.S. Pat. No. 5,146,771 to Loughlin shows a padlock with a shield into which the padlock may be inserted to deter forced attack. U.S. Pat. No. 4,567,740 to Kelly also shows a padlock provided with a housing for receiving and shielding the padlock. These padlocks, however, do not obscure a significant portion of the shackle, especially where it enters the padlock. Further, these shells are still susceptible to being pried away from the lock or allowing a prying member to attack the padlock directly or would necessarily have a loose fit, enabling the shell to slide off of the lock.
Therefore, the need still exists for a protective shell that can be easily, form-fittingly attached to a padlock, can obscure a portion of the shackle, can completely cover the underlying padlock, and contains seams that are not aligned with the seams of the padlock to further deter prying open of the padlock body.
Protection of the key holes of locks from elements has been provided by different mechanisms. For example, U.S. Pat. No. 2,874,563 to Schlage shows a keyhole shutter arranged in a housing that is to be attached to a door lock. The shutter itself includes opposed shutter-members that are coplanar and of the general form of segments of circles, having their straight edges in engagement. The periphery of the members is formed with a circumferential recess in which resides a C-shaped spring. In order to facilitate entry of a key between the edges, the edges may be chamfered inwardly.
Similarly, U.S. Pat. No. 952,643 to Roche discloses a spring loaded keyhole cover for a trunk-lock having segmental gates located in back of a circular opening formed in and concentric with the boss-like outer casing or shell of the lock. The outer shell confines the gates in a shallow concentric chamber formed between its inner face and the outer face of a corresponding boss-like inner shell. German Patent No. DT 1,780,397 to Hulsbeck discloses a keyhole cover having two semicircular halves meeting at a beveled common edge and a circumferential spring clip, and French Patent No. 638,078 to Hamon shows a spring loaded key hole cover.
Other locks have employed sliding keyhole covers, These sliding keyhole covers are shown, for example, in the Roche patent which also discloses a vertical and a horizontal sliding-plate gate. U.S. Pat. No. 116,972 to Madge shows a plate that can be slid over the key hole of a padlock, and U.S. Pat. No. 8,318 to Tilton discloses a padlock with a slide plate that can entirely cover or uncover the keyhole. U.K. Pat. No. 571,487 to Smith discloses a key-hole cover slidably engaged on a mounting plate that is attached to a padlock. In U.S. Pat. No. 1,062,594 to Isidor, a suitcase lock is provided with a escutcheon having a slidably disposed closing member with a knob to convert the keyhole, while U.S. Pat. No. 2,491,337 to Segal discloses a slidable guided latching member that has a circular head for covering the keyhole of a cylinder lock. U.S. Pat. No. 3,267,707 to Adams show a slide member having a disk-like body and a forwardly projecting rectangular knob portion that is held against the lock body by a retainer cap. The retainer cap includes a circular aperture, a vertical excursion to receive the knob, and a slide channel to receive the disk portion of the slide member.